In the applications that involve use of small brushless d.c. (BLOC) motors, for example, small-sized motors like the ones that can be used for hard-disk drives (HDDs), the amount of energy (current) that can be delivered by the supply source (battery or power supply) may be lower than the maximum one that can potentially be absorbed by the motor.
In such cases, it is hence possible to think of limiting the maximum current that is to supply the motor to levels that are tolerable for the supply source itself. At the same time, it is possible to act so as to cause the maximum current circulating in the coils of the motor not to exceed certain limit values.
In a switching driving circuit, current limitation may be obtained by limiting the operating duty-cycle of the power stage, for example, via dedicated circuits that use a feedback signal such as, for example, the current circulating in the load.
In this way, it is possible to control the maximum current circulating in the load (and hence circulating in the power stage), but not the current absorbed by the supply source.
In various situations, for example, when filter capacitances are present on the power supply line of the power-using device (the so-called “application”), it is possible to detect a difference between the current delivered by the power-supply circuit and the current circulating in the load. This difference between the two currents is a function of the operating duty-cycle of the power stage.
For instance, if the operating duty-cycle of the power stage is lower than 100%, the current that can be delivered by the supply source may be lower than the current absorbed by the load (which is of an inductive nature).
The mean value of the current delivered by the source may hence differ from the mean value of the current absorbed by the load, albeit preserving the energy balance in so far as the powers involved are the same: if the power supplied by the source is VCC*Iline (where VCC is the voltage of the supply source) and the power absorbed by the load is Vload*Iload where Vload=VCC*DC (where DC is the duty-cycle), the energy balance can be maintained in so far as VCC*DC*Iload=VCC*Iline.
For DC<100% the (mean) line current may, however, be lower (by a factor given by DC) than the (mean) load current. This difference may be quite considerable, which renders desirable the possibility of optimization of the global operation of the system by controlling both the current of the load and the current of the supply source.
In the case where the load is provided by a BLDC motor, control alone of the current of the motor, which may be used in various implementations, does not enable optimal exploitation of the energy that can be delivered by the supply source. The latter may thus be under-exploited, for example, during startup of the motor when the currents involved are high and the operating duty-cycle of the power stage may be lower than 100%.
For a simultaneous control of both of the currents (current of the motor and current of the supply source), it would be possible to hypothesise recourse to two distinct sensing elements, with a consequent increase in the cost of the application.
In applications where the aim is to reduce as much as possible the cost of the application—such as, for example, for hard-disk drives (HDDs)—it is possible to carry out current control not on the power supply line upstream of the filter capacitance/capacitances, but on the motor current. In some cases, it is possible to carry out control of the current on the power supply line, but downstream of the filter capacitance/capacitances. In this second case, it is, however, difficult to guarantee a good control of the current delivered by the supply source (line current).
To sense the current circulating in the motor (i.e., in the load) it is possible to envisage use of an external resistance (Rsense), i.e., of a sensing element integrated in the driving circuit (for example, sense-FETs), which enables minimization of the costs of the application.
Direct sensing of the effective current delivered by the supply source may use an element set in series to the power supply line and upstream of the filter capacitance. In some applications, such as, for example, HDDs, the cost of a sensing element upstream of the filter capacitances may not be negligible so that it appears preferable to carry out a control of current circulating in the motor (for example, via sense-FETs integrated in the power stage or else via a sensing resistance) or else to carry out a control of the line current downstream of the filter capacitances (for example, via sense-FETs integrated in the ISOFET, an element used for decoupling the motor from the power supply line). In HDD applications direct control of the effective current delivered by the supply source is not usually envisaged.